The present invention is related generally to machine manufacturing of components. In particular, the present invention is related to rapid prototyping manufacturing including layered manufacturing and solid freeform fabrication.
Using conventional techniques, a desired article to be made can initially be drawn, either manually or automatically utilizing a computer-aided design (CAD) software package. The article can be formed by removing material from material stock to form the desired shape in a machining operation. The machining operation may be automated with a computer-aided machining (CAM) process. The design and manufacture process may be repeated multiple times to obtain the desired part. A common practice is to mechanically remove material to create three-dimensional objects, which can involve significant machining skills and turn around time.
One process for making three-dimensional objects builds up material in a pattern as required by the article to be formed. Masters, in U.S. Pat. No. 4,665,492, discusses a process in which a stream of particles is ejected and directed to coordinates of the three-dimensional article according to data provided from a CAD system. The particles impinge upon and adhere to each other in a controlled environment so as to build up the desired article.
Processes and apparatus also exist for producing three-dimensional objects through the formation of successive laminae which correspond to adjacent cross-sectional layers of the object to be formed. Some stereo lithography techniques of this type use a vat of liquid photocurable polymer which changes from a liquid to a solid in the presence of light. A beam of ultraviolet light (UV) is directed to the surface of the liquid by a laser beam which is moved across the liquid surface in a single plane, in a predetermined XY pattern, which may be computer generated by a CAD system. In such a process, the successive layers may be formed in a single horizontal plane, with successive layers solidifying together to form the desired object. See, for example, U.S. Pat. No. 4,575,330 to Hull. Arcella et al., in U.S. Pat. No. 4,818,562, discuss a method for forming an article by directing a laser beam on a fusible powder which is melted by the beam, and which solidifies to form the desired shaped object.
Recently, various solid freeform fabrication techniques have been developed for producing three-dimensional articles. One such technique, used by Stratasys, Inc. (Eden Prairie, Minn.), is referred to as Fused Deposition Modeling (FDM). See, for example, U.S. Pat. No. 5,121,329 to Crump, herein incorporated by reference. FDM builds solid objects, layer by layer, from polymer/wax compositions according to instructions from a computer-aided design (CAD) software program. In one FDM technique, a flexible filament of the polymer/wax composition is heated, melted, and extruded from the nozzle, where it is deposited on a workpiece or platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the vertical Z direction. By controlling the processing variables, the extruded bead or xe2x80x9croadxe2x80x9d can be deposited layer by layer in areas defined by the CAD model, leading to the creation of the desired three-dimensional object.
Other examples of layered manufacturing techniques include multi-phase jet solidification techniques and/or laser-engineered net shaping. The extruded bead can be a ceramic suspension or slurry, a molten plastic, a powder-binder mixture, a polymeric material ready for curing or hardening, a molten metal, or other suitable materials which harden with time and/or exposure to an external stimulus. The bead can also be a curable strip of polymer or pre-polymer with polymerization initiated by radiation.
Conventional machining techniques utilize xe2x80x9csubtractivexe2x80x9d machining in which material is subtracted from a block of material. An example of subtractive machining is milling. Use of a subtractive computer controlled machine, such as a computer controlled milling machine, requires describing a tool path for the machine cutting element to follow in removing portions of the material stock. A cutting burr typically has a center axis and a known radius. A tool path for the cutting burr to follow is usually a series of line segments, line segment end points, or curves. The tool path may be generated according to a first rule that the innermost tool path stays about one burr radius from the surface perimeter to be formed. In an xe2x80x9cadditivexe2x80x9d manufacturing method, such as solid freeform manufacturing, the first rule from subtractive manufacturing has been informally adapted. The additive first rule is that the outermost tool path should come no closer than about half a bead width from the surface perimeter to be formed.
There are problems caused by a material depositing head following a tool path generated according to this first rule. Surface features that have a width less than a bead width cannot be entered by the depositing head, as the bead would extend outside of the surface perimeter to be formed. As a result, a narrow protrusion or vertex to be formed is not formed at all. This is contrary to the design intent.
A second, related rule for additive tool paths is that the tool path should not come closer than about half a bead width to an inner perimeter defining an interior feature. This rule prevents the path from filling in an interior feature, but can cause interior solid features of less than one bead width to be under-defined. In one example, two holes separated at their perimeters by less than a bead width will be formed as a single oblong hole, as the tool path cannot both follow the second rule and come between the two holes.
A third rule for additive tool paths is that the tool path should not cause the bead to cross the boundary of another bead, already generated from another tool path portion. As a contour, or outer perimeter following tool path may be more important, it is often generated first, to insure a surface closely resembling the design surface. In one case, often found in narrow parts, the inside surfaces of the contour tool path beads may come closer together than one bead width. A void will result at this location, as no tool path following along the contour tool path can enter this narrow region without violating the third rule.
Current processes for generating tool paths may include beginning at the outer perimeter and offsetting that perimeter inward into the material portion by about half the expected bead width. The resulting outer boundary can be used to define a contour tool path to define the limits for a raster tool path. In one situation, a first outer contour tool path may be offset within an outer perimeter vertex to create an outer boundary. If the outer boundary is used to form a contour bead to form the perimeter vertex, the interior of the contour bead may in turn form a second vertex, which may also present a problem in filling.
Another problem with existing technologies includes the creation of weak spots within the filled areas of solid slices made using raster filled layered manufacturing techniques. Yet another problem is the creation of perimeter gaps or sub-perimeter voids where raster tool paths meet perimeters or contour beads, respectively. What would be desirable are methods for generating tool paths that ameliorate some of the above-discussed deficiencies.
The present invention provides methods for improving the manufacture of objects made by layered manufacturing techniques through improved tool path generation. A vertex improvement aspect improves tool paths used to form vertices. Outer perimeter vertices can be improved by automatically creating an outer boundary reflecting the design intent to have material extending to the outer perimeter vertex. The outer boundary can be used as a contour tool path or as a limit to travel by raster tool paths. Boundary vertices within parts can be improved by extending more internal boundary vertices outward toward enclosing vertices, thereby eliminating some internal voids. Contour boundaries near outer perimeter corners can be better defined by extending outward a contour tool path toward the corners. Narrow regions between combinations of outer and/or inner perimeters can be filled through improved tool paths. Layer regions near inner voids can receive consistent filling through an improved raster tool path method.
One aspect of the invention improves the definition of designed perimeter vertices, for example, external protrusions, by creating an improved outer boundary. A conventional outer boundary is offset inward from the outer perimeter by about half the expected bead width to be deposited. The conventional offset method reduces the material extent at vertices. The present invention provides methods for creating an outer boundary vertex that is extended outward, toward the outer perimeter vertex. One set of methods accepts a conventional outer boundary as input, and can automatically relocate the vertices outward. Another set of methods creates a similar outer boundary, but without going through the intermediate step of generating a conventional outer boundary first. The outer boundaries can be used as contour tool paths or as limits to travel by raster tool paths.
The vertices created for internal tool paths can be improved by extending outward a more inner boundary vertex toward an enclosing, more outer boundary vertex. The more inner boundary vertex can be relocated to a center-to-center, boundary-to-boundary distance closer to the expected bead width. In one method, the more outer boundary is used as a contour tool path while the more inner boundary is used as a limit to travel by raster tool paths. In another method, both the more outer and more inner boundaries are used as contour tool paths. Reducing the internal, vertex-to-vertex distance can reduce or eliminate internal voids in parts.
Another aspect of the invention preserves internal designed features that are located close together. Internal voids located closer together than one bead width are not well defined by conventional tool path techniques because extending a tool path through the narrow region between the voids will extend past the offset boundaries around the void perimeters. The present invention allows a user to select relative weightings for preserving either or both void inner perimeters. Some embodiments provide improved tool paths in narrow regions between outer perimeters and inner void perimeters, which may be the case where inner voids are located near a part surface. Methods provided allow the user to create tool paths equidistant from each perimeter, or to create tool paths using methods such as medial axis transformations. Where one perimeter is to be given much greater weight, that perimeter offset boundary can be used to clip the other offset boundary or boundaries.
In yet another aspect of the invention, methods are provided which specify how raster tool paths are to be generated in the region near inner void perimeters. In one set of methods, a tool path is generated to follow at the offset distance around one side of void perimeter upon first intersection with the void offset boundary. Upon last intersection with the same offset boundary, a tool path is generated to follow at the offset distance around the opposite side of the void perimeter.
In one method, the tool path begins at an origin and attempts to travel away from the origin in a first axis direction along guide lines or in guide directions parallel to a second axis which can be perpendicular to the first axis. The tool path travel is constrained to travel along offset boundaries where they are intersected, to avoid invading the boundary interiors. When guide lines are intersected, or when certain increments of distance are reached in the first axis direction, travel can change to a direction parallel to the second axis, such as travel along a guide line. When the next guide line or increment of distance is reached, travel can continue in a reverse direction along the guide line. When an inner boundary is intersected for the first time, the tool path can travel along the boundary in a direction initially most toward the origin. When the inner boundary is intersected for the last time, the tool path can travel along the boundary in a direction initially most away from the origin. When a boundary is intersected intermediate the first and last times, travel can continue along the boundary away from the origin until the next guide line is reached, whereupon the travel direction is reversed relative to the last guide line travel.
In one aspect of the invention, raster tool path vertices can be automatically positioned such that gaps or sub-perimeter voids in between raster beads near an outer boundary can be eliminated or reduced. The raster beads can be considered formed as pairs having an outbound raster tool path segment heading toward the outer boundary, forming a first raster vertex heading into a turnaround segment which substantially parallels the outer boundary, and the turnaround segment forming a second raster vertex heading into an inbound segment heading away from the outer boundary. The gaps or sub-perimeter voids can be eliminated by positioning the first and second raster vertices such that they are disposed at the appropriate distance from the outer boundary.
In one set of methods, an outbound raster tool path from one pair is projected out to an outbound raster intersection point on the outer boundary. A locator, inter-pair line is also projected out to an intersection with the outer boundary. The adjacent, inbound raster tool path from the adjacent pair is projected out to an inbound raster intersection point on the outer boundary. If the outer boundary has no intermediate vertices between the respective raster tool path intersection point and the inter-pair intersection point, then the inter-pair intersection point is used as a location point to relocate the first and second original raster vertices.
If the outer boundary has such intermediate vertices, then these vertices are used in combination with the inter-pair intersection point to serve as a point to relocate the appropriate raster vertex. The intermediate vertices may be formed by the inwardly offset outer perimeter vertices. In one embodiment, raster vertices lie on a raster which has already been relocated at least once due to contour jogging as previously described. In this embodiment, the previously relocated vertices are not further relocated. Where the outer boundary is the outer perimeter, the intersection points between the outer perimeter and the projected raster lines and inter-pair lines can be used to position the raster vertices about half the raster bead width from the intersection points. When the outer boundary is effectively the inside of the contour bead, the intersection points can be used to position the raster vertices about half the raster bead width from the intersection points.